If you're wondering how these viruses get to be so enormous, you're not alone. As it turns out, that's the million-dollar question in the scientific community. "Nobody knows," says Van Etten. "There are some hypotheses, but certainly not one hypothesis that all scientists agree with," he explains, adding that some people believe that giant viruses have grown from smaller viruses, but nobody knows yet why they would have done that.
In fact, we've likely just begun to scratch the surface of giant viruses. "There are new ones being discovered all the time," Wilhelm says. "We really don't have any idea about the diversity, how many there are or how prevalent they are." He notes that at least one study has suggested they are mostly found in ocean environments. Scientists are very focused on attempting to find out why these genomes are so large. This is especially interesting because most normal viruses are actually under evolutionary pressures to reduce their genome size because it's so taxing for them to maintain high numbers, according to Wilhelm. "We call this genome streamlining," he explains. "Giant viruses go against the grain."
The makeup of giant viruses is also making waves in the virology community, since these big boys are sporting genes that were previously associated only with cellular life forms. For example, functional genes that are in charge of different processes, like nutritional transport, have been found in their depths. "We're starting to really blur the boundaries that we would have historically drawn between viruses and cells, in terms of function," explains Wilhelm.
As a result of these many findings, long-held opinions about viruses are being challenged. "It's clear that the field of virology has to re-think how they define viruses," says Van Etten, adding that previous standards held that, to qualify as a virus, it had to be able to pass through a filter of a certain size, and that this test is probably why the giant versions were unknown for so long.
Although many unknowns remain, a couple of principles are crystal-clear. "We're starting to expect that we're going to find relatives of these viruses everywhere," says Wilhelm. Also, the impact of all types of viruses is not to be underestimated. "Viruses control microbes, ergo viruses control the planet."